1. Field of the Invention
The present invention relates to a wiring board on which electrical elements such as semiconductor elements are mounted.
2. Description of the Related Art
There are known numerous products equipped with signal or power source wirings including functional elements such as semiconductor elements. Such known products include hybrid IC's and various IC packages. In recent days, new products have been developed in which an alumina wiring board or a printed wiring board provided with Ag-Pd wirings formed by screen printing is mounted in a so-called plastic package which utilizes lead frames and resin sealing and which is easy to mass produce and can be manufactured at low costs. These products can carry a plurality of elements.
Efforts are now being made to develop more highly integrated semiconductor devices which are lighter, thinner, shorter, smaller and less expensive. A wiring board which can meet such requirements and can be built into a plastic package is required. But, the above-described alumina wiring board has a problem in that it is difficult to increase its density and reduce its thickness beyond a certain point by reducing the wire diameter. The same is true with the printed wiring board. Also, the printed wiring board has another problem in that when mounting elements, especially when bonding with wires, the adhesive tends to soften.
An object of the present invention is to provide a wiring board which is free of the above problems and which satisfies the above requirements.
In order to solve the above problems, according to the present invention, there is provided a wiring board comprising a metal plate made of a lead frame material, a thin-film dielectric layer formed on the surface of the metal plate, an electric element being adapted to be mounted on the surface of the dielectric layer or on an exposed surface of the metal plate, and signal and/or power source wiring layers on the dielectric layer.
Each wiring layer should be in the form of (1) a laminate formed by laminating, by vapor phase deposition or by plating, a conductive layer of aluminum, an adhesive layer made of chromium, titanium or a laminate thereof, a diffusion barrier layer made of nickel, copper or a laminate thereof, and a corrosion-preventive and wire bonding layer made of gold, (2) a laminate formed by laminating, by vapor phase deposition or by plating, adhesive layer made of chromium, aluminum, titanium or a laminate comprising at least two of chromium, aluminum and titanium, a conductive layer of copper, and a corrosion-preventive and wire bonding layer made of gold, or (3) a laminate formed by laminating, by vapor phase deposition by plating, a conductive layer of aluminum, a diffusion barrier layer made of nickel, and a corrosion-preventive and wire bonding layer made of gold.
Aluminum and copper are metals available at low cost. Generally, gold is used for film wiring. Since gold is a precious metal, it is fairly expensive. Thus, by making the conductive layer from aluminum or copper, the material cost of the board can be reduced markedly. But, in case of the resin-sealed type of IC package, the wirings made of aluminum or copper tend to corrode due to the high hygroscopicity of resin.
Also, a gold wire bonding connection has a problem that intermetallic compounds tend to be produced due to thermal diffusion between gold and aluminum, thereby increasing the electrical resistance and reducing the strength at the connection. A gold-copper connection has a problem in that the bonding properties are not good. Moreover, wiring layers tend to peel off easily due to poor adhesion of the copper wiring layer to the dielectric layer.
According to the present invention, in order to prevent the corrosion of the wirings and to improve the adhesion when bonding with gold wires (by preventing the formation of compounds), an adhesive layer made of chromium, titanium or a laminate thereof is formed on an aluminum layer as a conductive layer, a diffusion barrier layer made of nickel, copper or a laminate thereof is formed further thereon, and a gold layer is further formed thereon. In the alternative, a diffusion barrier layer made of nickel is formed on an aluminum layer as a conductive layer, and a gold layer is further formed thereon.
If the conductive layer is made from copper, an adhesive layer made of chromium, aluminum, titanium or a laminate thereof is formed on the dielectric layer, a conductive layer of copper is formed thereon, and a corrosion-preventive and wire bonding gold layer is laminated further thereon.
These wirings should be formed by vapor phase deposition or by plating, because with these methods, the films can be miniaturized easily, i.e. the wiring width can be easily reduced to 100 microns or less, which is difficult with the screen printing method.
Considering the fact that the wirings have to have a conductivity of 100 m/mm or less, the aluminum or copper layer has to be fairly thick, i.e. 5 microns or more. But their mass-productivity is known to be very high.
The adhesive layer is provided to increase the adhesion between the aluminum conductive layer and the diffusion barrier layer or between the copper conductive layer and the dielectric layer. Its thickness should preferably be between 0.01 micron and 0.5 micron regardless of whether it is made of a single metal or a laminate of different metals. If 0.01 micron or less, sufficient adhesion is difficult to come by. If 0.5 micron or more, the cost for forming the film will become too high.
The thickness of the diffusion barrier layer should be within the range of 0.05 micron or more and 5 microns or less. If the thickness is 0.05 micron or less, no diffusion barrier effect will be manifested. If 5 microns or more, the film forming cost will become too high.
The thickness of the gold film layer should preferably be between 0.05 micron and 0.5 micron. If the thickness is 0.05 micron or less, it will be difficult to expect sufficient corrosion prevention or good wire bonding properties, If 0.5 micron or more, material costs will be too high, thereby increasing the cost of the end product.
The above-described arrangement can offer a wiring board having fine wirings and having good reliability even if it is used in the resin-sealed package. But by forming an inorganic dielectric film made of e.g. alumina as the dielectric layer directly on the metal board, a very thin surface mounting type flat package is obtained. Also, the reliability of the printed wiring board can be kept high because the adhesive layer never softens.